Ethanol has attracted attention as a fuel source alternative to petroleum fuels. The market scale of ethanol is estimated to be 55 million kiloliters for 2010. However, to adopt ethanol as a fuel, it is necessary to concentrate roughly distilled crude ethanol obtained from biomass such as corn to at least 99.5 wt % by dehydration.
One method for the dehydration is a pervaporation method using a water separation membrane as shown in FIG. 6(a). The pervaporation method refers to a method in which a target component such as water, for example, is removed in a gas state from a target fluid by using a membrane. In the pervaporation method, the temperature of a target fluid such as 95% roughly distilled ethanol is raised in a heat exchanger 61, and then the target fluid is fed to a feed side 62. The pressure on a permeation side 63, which is the other side of a water separation membrane, is reduced, creating a chemical potential difference produced across two sides of the water separation membrane 64. By this chemical potential difference, it is possible to cause the target component such as water, for example, to permeate from the feed side 62 to the permeation side 63. As a result, the pervaporation method makes it possible to remove, in a gas state, the target component such as water, for example, from the target fluid. Note that in FIG. 6(a), the reference numeral (65) denotes a condenser for condensing a permeate through the membrane, and the reference numeral (66) denotes a vacuum pump.
However, in the pervaporation method, if a water separation membrane with a high permeation rate of water, which is the separation target, is used, the evaporation of the permeated component significantly lowers the temperature of the target fluid. Therefore, in a case in which a water separation membrane with a high water permeation rate is used, the temperature of the target fluid on the feed side 62 significantly decreases as the target fluid goes from an inlet side A to an outlet side B as shown in FIG. 6b. In general, decrease in temperature of the target fluid significantly lowers the water permeation rate of a water separation membrane. When water separation membrane units are connected in series in the flow direction of a non-treated fluid, the temperature of the target fluid further decreases in the second water separation membrane unit, thereby significantly lowering the water permeation rates of the water separation membranes in and after the second water separation membrane unit.
In this connection, there has been proposed a use of a component not involving the above drawbacks (Patent Literature 1: Japanese Unexamined Patent Application Publication No. Hei 7-124444). Patent Literature 1 discloses a pervaporation membrane separation system in which at least one gas-liquid two-phase flow generating unit is provided upstream of pervaporation membrane separation units. This pervaporation membrane separation system includes multiple membrane separation units connected in series. In the pervaporation membrane separation system, a heat exchanger for maintaining the temperature of a raw material liquid is provided upstream of the first membrane separation unit to which the raw material is first fed.
However, when a non-treated fluid flows into a membrane unit as a gas-liquid two-phase flow, droplets of the non-treated fluid repeatedly collide with the surface of the membrane, thereby producing a problem of causing mechanical damage.
Moreover, it has been known that the boiling point of a target fluid decreases in a water separation membrane unit. This decrease in boiling point of a target fluid is probably caused by change in concentration of ethanol, pressure drop, or the like. The present inventors assumed that the boiling point of a target fluid is decreased by pressure drop.
Here, when the temperature of a target fluid exceeds the boiling point thereof, cavitation erosion occurs in the water separation membrane unit. The cavitation erosion refers to a phenomenon in which repeated events of generation and collapse of small bubbles due to local boiling phenomena damage materials by a high impact pressure generated when the bubbles collapse. For this reason, to prevent the water separation membrane units from being damaged, the target temperatures of the target fluid in the heat exchangers need to be set lower for further downstream water separation membrane units.
[Patent Literature 1] Japanese Unexamined Patent Application Publication No. Hei 7-124444